1. Field of the Invention
The present invention relates to an electro-optical device represented by an EL (electroluminescence) display device comprising a semiconductor element (an element using a semiconductor thin film, typically, a thin film transistor) that is formed on a substrate, and to an electronic device (electronic equipment) having the electro-optical device as a display. Specifically, the invention relates to a method of manufacturing these devices.
2. Description of the Related Art
In recent years, remarkable progress has been made in a technique for forming a thin film transistor (hereinafter referred to as a TFT) on a substrate, and developing the application of TFTs to active matrix type display devices is proceeding. TFTs using a polysilicon film, in particular, have a higher electric field mobility than that of conventional TFTs using an amorphous silicon film, and hence are capable of high speed operation. Accordingly, control of pixels, which in the past has been made by driver circuits outside the substrate, now can be made by driver circuits formed on the same substrate on which the pixels are formed.
The attention is kept drawn to such active matrix type display devices in expectation of various advantages, such as lowering of production cost, downsizing of display devices, and an increase in yield and throughput, as a result of putting all the various kinds of circuits and elements onto the same single substrate.
In active matrix type EL display devices, every pixel is provided with a switching element formed of a TFT, and an EL layer emits light when a drive element that controls current is operated by this switching element. The mainstream at present of the structure of the EL layer is the layered structure with three or four layers, proposed by Tang, et al. from Eastman Kodak Company.
Lately, however, many are concerned that multi-layering of the element structure will invite complication of the manufacture process and increase of the production cost. Attempts have been made in this connection to reduce the number of layers by doping a light emitting layer with a specific impurity element and using it as an electron conveying layer or an electron injection layer.
For instance, Idemitsu Kosan, Co. Ltd. proposes doping the light emitting layer (distylarylene derivative) near its surface with cesium (Cs) that has a small work function to use the doped region as the electron conveying layer. (Preliminaries for the Sixth FPD Seminar, sponsored by Electronic Journal, pp. 83-88)
The present invention has been made in view of the problem in multi-layering of the EL element, mentioned above, and an object of the present invention is therefore to provide a technique for reducing the number of the layers while keeping, or improving the performance of the EL element. More specifically, the present invention aims at providing a novel method of manufacturing an EL display device of the type that emits light from one side of a substrate where the element is formed.
Another object of the present invention is to reduce production cost so that an inexpensive EL display device is provided. Still another object of the present invention is to reduce product cost of an electronic device (electronic equipment) having the inexpensive EL display device as its display.
According to the present invention, a light emitting layer is doped with a specific impurity element to make the doped region function as a carrier conveying layer (or a carrier injection layer), thereby reducing the number of the layers in the light emitting layer of the EL element.
Specifically, the light emitting layer in the vicinity of an anode is doped with a halogen element in order to encourage the conveyance of holes. On the other hand, the light emitting layer in the vicinity of a cathode is doped with an alkali metal element or an alkaline earth metal element in order to encourage the conveyance of electrons. Ion doping that involves mass separation, or ion implantation that does not, may be used at the doping step of the impurity element.
The present invention is characterized in that doping of the above specific impurity element is carried out under a state in which no factors are found that might possibly invite degradation of the light emitting layer. To elaborate, the light emitting layer is doped with the specific impurity element while avoiding exposure to the open air (especially, oxygen). The conveyance of the holes or the electrons are thus improved without causing degradation of the light emitting layer.
As a typical example, an EL element consisting of a cathode, a light emitting layer and an anode layered in order from the bottom is doped with an impurity element through a passivation film formed on the anode. In this case, it is desirable that the process of from forming the cathode to forming the passivation film is completed without exposing the element to the open air not even once. An appropriate apparatus to achieve this is a thin film forming apparatus of multi-chamber system (also called cluster tool system) where various kinds of processing chambers are integrated, or a thin film forming apparatus of in-line system.